FIG. 10 is a cross-sectional view showing a conventional semiconductor package for packaging a solid-state imaging element in a solid-state imager. In FIG. 10, reference numeral 1 designates a solid state imaging element and reference numeral 2 designates a semiconductor package containing the solid-state imaging element 1. An internal wiring 21 of the semiconductor package 2 is provided on the surface of the package 2. Die-bonding resin 3 is used for fixing the solid-state imaging element 1 to the semiconductor package 2. Metal wires 4 are provided for electrically connecting the solid-state imaging element 1. External leads 5 provided at the side faces of the package 2 for electrically connecting the solid-state imaging element 1. Non-alkali glass 6 is provided for protecting the solid-state imaging element 1 and the metal wires 4 from the external atmosphere. Numeral 7 designates a sealing resin for fixing the non-alkali glass 6 to the semiconductor package 2. Numeral 8 designates radiation such as gamma rays.
A description is given of the structure of the semiconductor package of FIG. 10 hereinafter.
The solid-state imaging element 1 is fixed to the semiconductor package 2 by die-bonding resin 3 such as epoxy resin. Further, the internal wiring 21 and the pad of the solid-state imaging element 1 are connected by the metal wires 4 and thus the electric signals from the solid-state imaging element 1 are transferred outside of the package 2 by the external leads 5.
Further, in order to protect the solid-state imaging element 1 from the external atmosphere, including radiation such as gamma rays 8, non-alkali glass 6 is fixed to the semiconductor package 2 using sealing resin 7.
The incident light 8 incident from above the non-alkali glass 6, passes therethrough and is received by the receiving surface of the solid-state imaging element 1 and then an electrical signal from the solid-state imaging element 1 is taken to the outside through the external leads 5.
FIG. 11 shows an optical characteristics of the non-alkali glass where radiation such as gamma rays 8 irradiated to the solid-state imaging element 1 packaged as shown in FIG. 10. As shown in the FIG. 11, although the non-alkali glass 6 has transmissivity of about 100% for light of wavelength 400 nm before the irradiation by gamma rays (shown as "Pre-Rad" in FIG. 11), as the irradiation amount of gamma rays becomes 4.times.10.sup.4 rads, 3.times.10.sup.5 rads, and 1.times.10.sup.6 rads, the transmissivity of the non-alkali glass is gradually reduced. This is because the color of the non-alkali glass changes and becomes non-transparent due to irradiation by gamma rays.
The conventional semiconductor package for a solid-state imager is constructed as described above, and when it is in a space environment, especially mounted on an artificial satellite or used in observations for a nuclear reactor, it is directly exposed to radiation such as gamma rays, and the non-alkali glass is deteriorated.
Furthermore, this radiation generates holes that are trapped in the gate oxide film of the n channel transistor constituting the solid-state imaging element 1 and the threshold voltage therefore varies.
White circles in FIG. 4 show the change of the threshold voltage of the transistor of the solid-state imaging element 1 due to irradiation by gamma rays where quartz is used for the non-alkali glass 6. As shown in FIG. 4, the threshold voltage which is 0.4 V before the irradiation, becomes about -0.8 V after the irradiation by gamma rays of 10.sup.6 rads. Such a large change in threshold voltage deteriorates the transistor characteristics and deteriorates the reliability of the solid-state imager.
Furthermore, because the non-alkali glass 6 is only fixed to the upper surface of the semiconductor package itself 2 by sealing resin 7, it may get out of position with respect to package 2 and cause leakage of air into the package.